Nowadays, there is a demand for improving the fuel efficiency of automobiles in order to control CO2 emission from the viewpoint of the global environment conservation. In addition, there is a demand for improving the safety of automobiles including the crashworthiness of the automobiles in order to achieve satisfactory occupant safety at the time of a crash. Therefore, there is an active trend toward decreasing the weight of automobiles and increasing the strength of automobiles.
It is considered that decreasing the weight of automobiles through a decrease in the thicknesses of materials for parts within a range in which there is no problem regarding rigidity as a result of increasing the strength of the materials for parts is effective in order to realize a decrease in the weight of automobiles and an increase in the strength of automobiles at the same time. Nowadays, there is an active trend toward using a high-strength steel sheet for automobile parts so that the tensile strength (TS) of steel sheets which are used for the structural members and stiffening members of automobiles reaches 980 MPa or more, and a steel sheet having a TS of 1180 MPa or more is also used. The utilization of microstructure strengthening is effective for increasing the strength of a steel sheet. In particular, a multi-phase steel sheet composed of soft ferrite and hard martensite generally has good ductility and an excellent strength-ductility balance and is a kind of strengthened steel sheet having a comparatively good press formability. However, in the case of such a multi-phase steel sheet, since the mechanical properties of the steel sheet such as tensile strength (TS) widely vary due to a variation in conditions such as annealing temperature which occurs when the multi-phase steel sheet is manufactured by using an ordinary continuous annealing line, the mechanical properties tend to vary in the longitudinal direction of a coil, that is, in the longitudinal direction of the steel sheet wound in a coil shape. Since it is difficult to stably perform press forming in a continuous pressing line for automobiles due to such deviation of mechanical properties, there is a risk of a significant decrease in usability. In addition, when the strength of a steel sheet is increased, there are an increase in the content of Si, which is a solid solution chemical element effective for increasing strength and an increase in the contents of, for example, C and Mn, which are added to achieve a necessary amount of martensite in order to increase strength. Since Si and Mn are easily oxidizable chemical elements which are more readily oxidized than Fe, achieving satisfactory zinc coatability and surface appearance quality is an issue in the case where a galvanizing treatment is performed on a steel sheet containing large amounts of Si and Mn. That is, since Si and Mn contained in steel are subjected to selective oxidation even in a non-oxidizing atmosphere or a reducing atmosphere used in a general annealing furnace, Si and Mn are concentrated and form oxides on the surface of the steel, which may cause a coating defect occurring due to a decrease in the wettability of molten zinc to a steel sheet when a galvanizing treatment is performed.
Patent Literature 1 proposes a method for improving the wettability with molten zinc in order to improve the adhesiveness of a galvanizing layer in which an Fe oxide film is formed rapidly on the surface at an oxidation speed higher than a specified speed by heating a steel sheet in an oxidizing atmosphere in advance in order to prevent the oxidation of additive chemical elements such as Si and Mn on the surface of the steel sheet and in which the Fe oxide film is then reduced by performing annealing in a specified atmosphere. In addition, Patent Literature 2 proposes a method in which the surface-concentration matter of easily oxidizable chemical elements such as Si and Mn, which are concentrated on the surface of a steel sheet, is removed by performing pickling on the steel sheet after annealing has been performed, in which annealing is then performed again, and in which a galvanizing treatment is then performed.